Purification of precipitates.



DEAgNE BURNS, 01? LAKEWOOD, OHIO, AssreNon, BY MESNE ASSIGNMENTS, To

' I NATIONAL CARBON COMPANY, INC., A CORPORATION OF NEW YORK.

PURIFICATION OF PBECIPITATES.

useful ImprOvementin the Purification of Precipitates, of which the following is a full, clear, and exact description.

My invention in its broad aspect relates to the elimination of occluded material from a precipitate, and the particular application of the process as described herein relates to .a method of treating sulfates of a rare-earth metal to convert it into fiuorids for use in arc lamp electrodes. As 1s well known,

- these sulfates are secured by the treatment of monazite sands in the process of extracting thorium compounds for Welsbach mantles. The material remaining after this treatment is known to the trade as Welsbach or cerium residue. This residue consists principally of cerium salts mixed with other salts of rare-earth metals together with impurities.

The prior process of obtaining fluorids of rare-earth metals has been to treat the cerium and other sulfates of the VVelsbach residue with hydrofluoric acid to precipi-\ tate lluomds of rare-earth metals. This treatment, however, produces a product with only about 23% of fluorin content, whereas the theoretical amount should be almost 29%. In addition to this, there IS approximately 3% of sulfates that cannot be washed out. I have not been able to definitely determine the chemical form of this sulfate, but am inclined to think that it is occluded sulfuric acid, though it may be a sulfate of some metal constituent of the mixture, such 'as a sulfate of a rare-earth metal for instance.

The sulfate content 1n 'fluorids of rareearth metals seems to make the product hygroscopic, which is objectionable. It is necessary to calcine the fiuorids in order to make the product dense and non-shrinking when v I incorporated in electrodes, and it the product 'contalns water a reaction Wlll be set up during calcination in accordance with the following equation 2REF,+3H,O:RE,O +6HF,

where RE stands for various rare earth metals with cerium as the principal constituent. This means that if the fluorids of rareearth metals are hygroscopic to any extent a Specif cation Of Letters Patent.

Patented July 16:, 1918.

Application filed September 19, 1914:, Serial No. 862,561.

'definiteamount of rare-earth oxid will be lent amount of hydrofluoric'acid liberated.

Inasmuch as fluorid compounds of the rareearth metals are more efficient and suitable for are lamp electrodes, it is objectionable to have any considerable part of them changed over into the oxid in their preparation.

The sulfate content of the flu'orids also has another objectionable feature when used in cored electrodes in which silicates of sodium or potassium compose the binding medium, as it causes the silicates to gelatinize to such an extent that the core paste cannot be readily squirted into the carbon shells.

My invention is directed toward the eliminationof the sulfates from. mixtures of the fiuorids of the rare-earth metals and to the increasing of the fluorin content. The process by which these results are accomplished will now be described.

A solution of sulfates of rare-earth metals,

.which may be obtained by extraction from is thengradually calcined to a temperatureranging between 500 C. and 800 (7., prefel'ablywith exclusion of air. In practice it has been found that the preferable temperature is between 050 and 090. \Vllen fiuorids of the rare-earth metals are precipitated they contain about 3% of sulfates, as S0 that cannot be washed out. The sodium fluorid reacts on these in accordance with the following equation:

where m represents generally the positive radical. I This reaction probably takes place slowly at ordinary temperatures, but when the product is calcined the reaction is complete. From the above equation it willbe seen that in case the sulfate is in the form of sulfates Ofl'ttlG-Btll'tll metals, fluorids of rare-earth metals will be formed which will further increase the fluorin content. The calcined product is crushed and Washed with water to remove the soluble sodium sulfate. After the mass has been washed it is dried at about 110 C. and is ready for use in electrode mixes.

While I have described the preferred process for bringing about the desired reaction, this may 'be modified. The sulfates of rare-earth metals solution may be added to an excess of sodium fluorid solution to to precipitate the fluorids of rare-earth metals.

. The precipitate is then washed incompletely so that there remains enough sodium fluorid solution mixed with the sludge to bring about the reaction which results inzthe ultimate removal of the sulfate content as dee scribed above.

It also would not be necessary to add the correct amount of sodium fiuorid as described in the preferred process, but an excess may be added to the sludge before it is filter pressed. This leaves more than enough sodium fluorid to bring about the reaction.

The dry fluorids of rare-earth metals may also be mixed with dry sodium fluorid and ground together to intimately mix the same.

The mass is then calcined as previously described to bring about the reaction.

While I have described the use of sodium fiuorid, potassium liuorid works equally as well, though it is not so cheap. ()ther alkali liuorids can also be used, as the invention essentially consists in the use of a soluble fiuorid that will react on the impurity to transform it into a soluble salt so that it can be washed out.

My invention is not only useful in pre: paring mixes forarc lamp electrodes,.but also is of advantage in preparation of metallic cerium for pyrophoric alloys. 'In such 40 process cerium is produced by electrolyzing fused cerium or other fiuorids of rare-earth metals. If thereis any water contained in the fluorids it will be driven oil" in the form of steam which reacts with the fluorids to form oxid of rare-earth metals and hydro fiuoric acid. This has been found so objectionable that in the past the water has been removed from the cerium use of absolute alcohol. As is well known, absolute alcohol is very expensive, and by using my process for the formation of cerium fluorid a non-hygroscopic anhydrous fluorid will be obtained at much less cost. If pure metallic cerium-is desired it will, of course, be understood that salts of this mafluorid by the yterial will be separated from the other constituents of the Welsbach residue by known means. Since cerium is the major part of the VVelsbach residue it may be unnecessary to make this separation.

There is a known way for forming fluorids of rare-earth metals free from sulfates which consists in treating the extract from Wels- I soluble salts thus formed.

bach residue with a carbonate to form in-j soluble rare-earth carbonates from which the sulfates can be readily washed. The carbonates are. then treatedwith boiling hydrofluoric acid to form the fluorids of rare-earth metals. This process requires an'additional step and is otherwise not so satisfactory as the safe handling of boiling hyrofiuoric acid is a diflicult problem. The fiuorids obtained by precipitation are dense and almost glassy in condition and appearance, while those obtained by treating an insoluble compound of rare-earth metals, such as the carbonate,

with boiling hydrofluoric acid are light'and earthy in appearance. The dense precipitated form is much better for trodes.

While I have explained the complete process of obtaining non-hygroscopic fiuor-ids of rare earth metals, free from sulfates, direct from the \Velsbach residue, it will, of course, be apparent that the invention also includes the use of sodium or other fluorid to free fluorids of. rare-earth metals from sulfates regardless of where the fiuorids are obtained.

arc lamp elecplication of heat, and applying heat to bring about the reaction 2. The steps in the process of reparing fiuorids of rare-earth metals whic consists in adding a solution of a soluble. salt of a rare-earth metal to hydrofluoric acid, drying the precipitate,

3. The steps in the process of freeingpreciptated fluoridsof rare-earth metals of another occludedrare-earth compound, which consists in mixing them with sodium fluorid and calcining to a temperature, above 500 C. 4. The process of eliminating sulfates from fluorids of rare-earth metals, which consists in mixing said fluorid, calcining b 11 it to a temperature above su stantia y 500 C. and

In testimony whereof, I hereunto affix my signature. v

DEANE BURNS. Witnesses:

H. G. GR'OVER,

L. E. WRIGHT.

mixing it with afiuorid of an alkali metal and-calcining it to above washing out the fluoride-with sodium 

